1. Field of the Invention
The present invention relates to a load sensing drive force transmission device for converting rotational motion of a male screw member into linear movement of a female screw member, and whereby when a drive force is imparted to an object, the drive force is automatically adjustable in accordance with a load from the object.
2. Description of the Related Art
A conversion device is known, for example, which utilizes a screw mechanism in which a female screw member is screw-engaged with a male screw member that is connected to a drive shaft, and wherein rotational motion of the male screw member is converted into linear movement of the female screw member, for converting rotational motion from an electric motor or the like into linear movement and for imparting a drive force to an object.
In an ordinary screw mechanism, the transmission ratio between the male screw member and the female screw member is fixed, such that a large reduction ratio must be set beforehand in order to impart a large drive force to the object. However, when the reduction ratio is made large, even when the load is light, because the movement velocity of the female screw member is slow, a long time is required until the desired drive force can be attained.
Accordingly, a load sensing type drive force transmission device has been developed, in which the reduction ratio can be automatically switched responsive to loads from the object (see, Japanese Patent No. 3664406, the disclosure of which is expressly incorporated into the present specification by reference).
FIG. 5 is a cross sectional structural view of a conventional load sensing drive force transmission device 2. The load sensing drive force transmission device 2 is equipped with a male screw member 8 having one end thereof fixed to a rotational shaft 6 pivotally supported by an upper frame 4, a female screw member 10 screw-engaged with the male screw member 8, a movable case 14 attached via bearings 12a, 12b to the female screw member 10, and a pressing plate 18 affixed to the movable case 14 via a sleeve 16 through which the other end of the male screw member 8 passes.
The male screw member 8 is supported so as to be rotatable within the movable case 14 through bearings 20a, 20b. The female screw member 10 is set to have a larger effective diameter than the male screw member 8, such that the axis thereof is offset a predetermined amount with respect to the axis of the male screw member 8, and the female screw member 10 is rotatably supported in the movable case 14 through the bearings 12a, 12b. The female screw member 10 is movable with respect to the movable case 14. One end of the female screw member 10 is connected to the movable case 14 through brake shoes 22a, 22b, whereas the other end is connected to the movable case 14 through a spring 24.
In the load sensing drive force transmission device 2 which is constructed in this manner, when the male screw member 8 is rotated accompanying rotation of the rotational shaft 6, the female screw member 10, which is screw-engaged with the male screw member 8, is moved in the direction of the arrow, whereby the movable case 14 and the pressing plate 18 via the sleeve 16 also are moved in the direction of the arrow.
The pressing plate 18 applies a predetermined pressing force with respect to an object. When the load from the object with respect to the pressing plate 18 is at or below a predetermined value, because the female screw member 10 is pressed upon by the movable case 14 through the brake shoes 22a, 22b in accordance with a resilient force of the spring 24, the female screw member 10 does not rotate and continues to move toward the object together with the movable case 14. Accordingly, the movement velocity of the movable case 14 is determined by the pitch of the male screw member 8 and the female screw member 10.
When the pressing plate 18 has moved a predetermined amount and the load from the object becomes larger than the predetermined value, movement of the movable case 14 in the direction of the arrow is prevented by the object. At this time, upon rotation of the male screw member 8, the female screw member 10 accommodated inside the movable case 14 moves in the direction of the arrow relatively with respect to the movable case 14 against the resilient force of the spring 24, and accordingly the brake shoes 22a, 22b separate therefrom and the female screw member 10 attains a state in which it becomes rotatable with respect to the movable case 14.
In this case, because the effective diameter of the female screw member 10 is set to be larger than that of the male screw member 8, the female screw member 10 rotates at a reduction ratio determined in accordance with the ratio of their effective diameters. Accordingly, the movement velocity of the movable case 14 in the direction of the arrow is decelerated, and a large drive force is imparted to the object.
Incidentally, with the load sensing drive force transmission device 2 constructed as described above, when the brake shoes 22a, 22b separate away from each other, because the female screw member 10 is placed in a condition where it can be freely rotated through the bearings 12a, 12b, the reduction ratio and the drive force increase abruptly, with the disadvantage that shocks caused by this change are imparted to the object.
Further, in the case that the pressing plate 18 is moved from the state illustrated in FIG. 5 in a direction opposite to the arrows, since the brake shoes 22a, 22b do not separate away from each other, the reduction ratio and the drive force cannot be automatically adjusted corresponding to loads from the object. Accordingly, another disadvantage exists in that the direction in which the reduction ratio and the drive force are capable of being adjusted is restricted to one direction only.